The present disclosure relates to a separator including a substrate and a surface layer which compensates the strength thereof and improves a tear strength as a whole and a nonaqueous electrolyte battery using the separator. In addition, the present disclosure also relates to a battery pack, an electronic apparatus, an electric vehicle, an electric power storage device, and an electric power system, each of which uses the nonaqueous electrolyte battery described above.
In recent years, along with the popularization of portable information electronic apparatuses, such as a mobile phone, a video camera, and a notebook personal computer, improvement in performance, reduction is size, and reduction in weight of these apparatuses have been pursued. Although disposable primary batteries and rechargeable secondary batteries have been used as power sources of these apparatuses, in consideration of good comprehensive balance among high performance, compact size, light weight, economical efficiency, and the like, the demand of secondary batteries, in particular, a lithium ion secondary battery, has been increasing. In addition, in the apparatuses described above, further improvement in performance, reduction in size, and the like have been continuously performed, and also as for the lithium ion secondary battery, an increase in energy density has been demanded.
Since the energy density of the lithium ion secondary battery is increased concomitant with an increase in capacity thereof, improvement in reliability in the case in which large energy is released at the time of battery overheating or internal short circuit has also been increasingly demanded. Accordingly, a lithium ion secondary battery which can simultaneously satisfy high reliability in the cases as described above and an increase in capacity has been strongly desired.
A common lithium ion secondary battery includes a positive electrode containing a lithium composite oxide, a negative electrode containing a material capable of occluding and releasing lithium ions, at least one separator provided between the positive electrode and the negative electrode, and a nonaqueous electrolyte. In addition, the positive electrode and the negative electrode are laminated with the separator provided therebetween, or after being laminated with the separators provided therebetween, the positive electrode and the negative electrode are wound to form a columnar wound electrode. The separator functions to electrically isolate between the positive electrode and the negative electrodes and to retain a nonaqueous electrolyte. As the separator for the lithium ion secondary battery as described above, a porous film formed from a resin material, such as a polyolefin resin, has been commonly used.
In the past, in order to simultaneously obtain heat resistance and high strength, the separators have been frequently formed by a so-called wet method. The wet method is a method for forming a porous film by the steps of mixing a resin material forming a separator with a plasticizer or the like, forming a thin film, for example, by extrusion using a melt extrusion method, and extracting the plasticizer from the film to form pores at places at which the plasticizer is present. Since the pores are formed isotropically in the separator formed by the wet method, the tear strength is also isotropic. In addition, in a battery using the separator formed by the wet method, effects of performing a uniform electrochemical reaction, suppressing precipitation of lithium dendrites, and the like can also be obtained.
However, for the formation of the separator by the wet method, blending and extraction of the plasticizer are necessary, and in order to form a porous film having fine and uniform pore diameters, an operation process is not only complicated but also, for example, a treatment of an extraction liquid is inevitably performed. In addition, because of the complicated manufacturing process, its cost becomes very high.
Accordingly, a technique has been proposed in which a porous film formed using a so-called dry method is used as the separator. The dry method is a method for forming a porous resin film by forming a crystallized polyolefin resin into a thin film, for example, by a melt extrusion method, then performing an annealing (heating) treatment on the film, and drawing the film in one direction (uniaxial drawing). The dry method includes no extraction step (that is, no solvent treatment) of extracting a plasticizer or the like which is performed in the wet method. Therefore, in the dry method, a porous film can be formed by a substantially simple process as compared to that of the wet method. Accordingly, a separator formed by the dry method is generally inexpensive as compared to that formed by the wet method. On the other hand, a porous film formed by uniaxial drawing has a problem in that the tear strength in a drawing direction (Machine Direction, hereinafter referred to as “MD direction”) is high and that in a direction perpendicular to the drawing direction (Transverse Direction, hereinafter referred to as “TD direction”) is low. For this reason, the tear strength has anisotropy, and when a tensile load is applied in a drawing direction, the film is liable to be torn.
In addition, a separator having an inferior tear strength is easily torn when a foreign material is present in a separator formation process, and as a result, the yield is decreased. In addition, since the separator as described above is liable to be torn by a foreign material mixed in a battery, a dropping impact, or the like, the battery has a problem in safety.
Accordingly, Japanese Unexamined Patent Application Publication No. 2005-343937 has proposed a separator which contains three components, that is, a high-density polyethylene resin, a polypropylene resin, and a filler, and which has pores formed starting from the filler by drawing in one direction. Japanese Unexamined Patent Application Publication No. 2005-343937 has intended to improve tearing resistance of the separator itself by mixing a high-density polyethylene resin and a polypropylene resin.
In addition, Japanese Unexamined Patent Application Publication No. 2010-111096 has disclosed that a laminated porous film which has a layer containing a polypropylene resin as a primary component and a layer containing a polyethylene resin and which has a β activity is used as a separator. In Japanese Unexamined Patent Application Publication No. 2010-111096, since the layer containing a high-melting-point and a high-strength polypropylene resin as the primary component and a layer containing a soft and a low-melting point polyethylene resin are laminated to each other, a separator capable of performing shutdown at a predetermined temperature can be obtained while the strength thereof is maintained. In addition, in Japanese Unexamined Patent Application Publication No. 2010-111096, in order to obtain the balance of mechanical properties, the ratio in tear strength in the TD direction to the MD direction is set in a predetermined range.
Furthermore, Japanese Unexamined Patent Application Publication No. 2007-277580 has proposed as a separator having a high tear strength, a separator including a substrate formed of at least one of fiber and pulp and a porous layer which contains a para-aramide polymer covering the substrate and filling pores in the substrate.